1. Field of the Invention
In order to reduce friction and increase the load-carrying capacity of lubricants, especially when employed under extreme-pressure conditions, additives are used which have come to be known as "EP" (extreme-pressure) agents. At high loads, these additives prevent the welding of irregularities in the two contacting surfaces which might otherwise occur, followed by metal transfer (galling) or cleavage and the production of wear fragments. A commonly used additive is a zinc dithiophosphate which is the dithiophosphate ester of dithiophosphoric acid and a hydrocarbyl alcohol. However, as the search for improved EP additives continues, alternatives to zinc dithiophosphates have been sought.
2. Description of the Prior Art
U.S. Pat. No. 3,547,820 discloses a lubricating composition containing the metal salt of a phosphate ester of an oxyalkylated hydroxy compound. U.S. Pat. No. 3,791,985 discloses a metal salt of sulfur-containing phosphate esters of alkoxylated phenols in lubricating compositions.